This invention relates in general to transmissions for selectively providing a desired speed reduction gear ratio between an input shaft and an output shaft. In particular, this invention relates to a system for supporting the shift rails and shift forks for selective sliding movement in a manually shifted vehicular transmission.
In virtually all land vehicles in use today, a transmission is provided in a drive train between a source of rotational power, such as an internal combustion or diesel engine, and the driven axle and wheels of the vehicle. A typical transmission includes a case containing an input shaft, an output shaft, and a plurality of meshing gears. Means are provided for connecting selected ones of the meshing gears between the input shaft and the output shaft to provide a desired speed reduction gear ratio therebetween. The meshing gears contained within the transmission case are of varying size so as to provide a plurality of such gear ratios. By appropriately shifting among these various gear ratios, acceleration and deceleration of the vehicle can be accomplished in a smooth and efficient manner.
Many transmission structures are known in the art for performing these gear ratio selections manually, i.e., in response to some physical exertion by the driver of the vehicle. In a conventional manual transmission, the driver grasps and moves an upper portion of a pivotable shift lever to effect shifting of the transmission. In response thereto, a lower portion of the shift lever engages and moves one of a plurality of shift rails provided within the transmission. The shift rails are typically supported within the transmission case for sliding movement from a central neutral position either axially forwardly to a first gear engaging position or axially rearwardly to a second gear engaging position. Thus, the selection and movement of a particular shift rail causes certain ones of the meshing gears to be connected between the input shaft and the output shaft. As a result, a desired gear ratio is provided between the input shaft and the output shaft. Manually operated transmissions of this general type are well known in the art and are relatively simple, inexpensive, and lightweight in structure and operation. Because of this, the majority of medium and heavy duty truck transmissions in common use today are manually operated.
Typically, the shift rails extend longitudinally throughout the transmission and are supported at their ends by a pair of bearing blocks. The shift rails can be individually selected and longitudinally shifted by a shift lever so as to provide a desired one of a plurality of gear ratios between the input shaft and the output shaft of the transmission. To accomplish this, each of the shift rails has a shift fork secured thereto. The shift forks extend downwardly from the shift rails into engagement with respective clutches contained within the transmission. As is well known, axial movements of the shift rails (and the shift forks secured thereto) cause the associated clutches to selectively connect the appropriate gears contained within the transmission to drive the output shaft at a desired speed reduction gear ratio relative to the input shaft.
Usually, mechanical fasteners, such as rivets, are used to secure the shift forks to their associated shift rails. Thus, longitudinal movement of a shift rail causes corresponding longitudinal movement of its associated shift fork. Although this structure has been effective, it has been found that the rivets used to secure the shift forks to the shift rails can work loose during use, causing premature failure. Thus, it would be desirable to provide an improved structure for supporting the shift rails and shift forks for selective sliding movement in a vehicular transmission.